1 Introduction

This document serves as the complete definition of Google's coding standards for
source code in the Java™ Programming Language. A Java source file is described as being in
Google Style if and only if it adheres to the rules herein.

Like other programming style guides, the issues covered span not only aesthetic issues of
formatting, but other types of conventions or coding standards as well. However, this document
focuses primarily on the hard-and-fast rules that we follow universally, and
avoids giving advice that isn't clearly enforceable (whether by human or tool).

1.1 Terminology notes

In this document, unless otherwise clarified:

The term class is used inclusively to mean an "ordinary" class, enum class,
interface or annotation type (@interface).

The term comment always refers to implementation comments. We do not
use the phrase "documentation comments", instead using the common term "Javadoc."

Other "terminology notes" will appear occasionally throughout the document.

1.2 Guide notes

Example code in this document is non-normative. That is, while the examples
are in Google Style, they may not illustrate the only stylish way to represent the
code. Optional formatting choices made in examples should not be enforced as rules.

2 Source file basics

2.1 File name

The source file name consists of the case-sensitive name of the top-level class it contains,
plus the .java extension.

2.2 File encoding: UTF-8

Source files are encoded in UTF-8.

2.3 Special characters

2.3.1 Whitespace characters

Aside from the line terminator sequence, the ASCII horizontal space
character (0x20) is the only whitespace character that appears
anywhere in a source file. This implies that:

All other whitespace characters in string and character literals are escaped.

Tab characters are not used for indentation.

2.3.2 Special escape sequences

For any character that has a special escape sequence
(\b,
\t,
\n,
\f,
\r,
\",
\' and
\\), that sequence
is used rather than the corresponding octal
(e.g. \012) or Unicode
(e.g. \u000a) escape.

2.3.3 Non-ASCII characters

For the remaining non-ASCII characters, either the actual Unicode character
(e.g. ∞) or the equivalent Unicode escape
(e.g. \u221e) is used, depending only on which
makes the code easier to read and understand.

Tip: in the Unicode escape case, and occasionally even when actual Unicode characters
are used, an explanatory comment can be very helpful.

Examples:

Example

Discussion

String unitAbbrev = "μs";

Best: perfectly clear even without a comment.

String unitAbbrev = "\u03bcs"; // "μs"

Allowed, but there's no reason to do this.

String unitAbbrev = "\u03bcs";
// Greek letter mu, "s"

Allowed, but awkward and prone to mistakes.

String unitAbbrev = "\u03bcs";

Poor: the reader has no idea what this is.

return '\ufeff' + content;
// byte order mark

Good: use escapes for non-printable characters, and comment if necessary.

Tip: Never make your code less readable simply out of fear that some programs might
not handle non-ASCII characters properly. If that should happen, those programs
are broken and they must be fixed.

3 Source file structure

A source file consists of, in order:

License or copyright information, if present

Package statement

Import statements

Exactly one top-level class

Exactly one blank line separates each section that is present.

3.1 License or copyright information, if present

If license or copyright information belongs in a file, it belongs here.

3.2 Package statement

The package statement is not line-wrapped. The column limit (Section 4.4,
Column limit: 80 or 100) does not apply to package statements.

3.3 Import statements

3.3.1 No wildcard imports

Wildcard imports, static or otherwise, are not used.

3.3.2 No line-wrapping

Import statements are not line-wrapped. The column limit (Section 4.4,
Column limit: 80 or 100) does not apply to import
statements.

3.3.3 Ordering and spacing

Import statements are divided into the following groups, in this order, with each group
separated by a single blank line:

All static imports in a single group

com.google imports
(only if this source file is in the com.google package
space)

Third-party imports, one group per top-level package, in ASCII sort order

for example: android, com, junit, org,
sun

java imports

javax imports

Within a group there are no blank lines, and the imported names appear in ASCII sort
order. (Note: this is not the same as the import statements being in
ASCII sort order; the presence of semicolons warps the result.)

3.4 Class declaration

3.4.1 Exactly one top-level class declaration

Each top-level class resides in a source file of its own.

3.4.2 Class member ordering

The ordering of the members of a class can have a great effect on learnability, but there is
no single correct recipe for how to do it. Different classes may order their members
differently.

What is important is that each class order its members in some logical
order, which its maintainer could explain if asked. For example, new methods are not
just habitually added to the end of the class, as that would yield "chronological by date
added" ordering, which is not a logical ordering.

3.4.2.1 Overloads: never split

When a class has multiple constructors, or multiple methods with the same name, these appear
sequentially, with no intervening members.

4 Formatting

Terminology Note:block-like construct refers to the body of a class, method or
constructor. Note that, by Section 4.8.3.1 on
array initializers, any array initializer
may optionally be treated as if it were a block-like construct.

4.1 Braces

4.1.1 Braces are used where optional

Braces are used with
if,
else,
for,
do and
while statements, even when the
body is empty or contains only a single statement.

Line break after the closing brace if that brace terminates a statement or the body
of a method, constructor or named class. For example, there is no line break
after the brace if it is followed by else or a
comma.

A few exceptions for enum classes are given in Section 4.8.1,
Enum classes.

4.1.3 Empty blocks: may be concise

An empty block or block-like construct may be closed immediately after it is
opened, with no characters or line break in between
({}), unless it is part of a
multi-block statement (one that directly contains multiple blocks:
if/else-if/else or
try/catch/finally).

Example:

void doNothing() {}

4.2 Block indentation: +2 spaces

Each time a new block or block-like construct is opened, the indent increases by two
spaces. When the block ends, the indent returns to the previous indent level. The indent level
applies to both code and comments throughout the block. (See the example in Section 4.1.2,
Nonempty blocks: K & R Style.)

4.3 One statement per line

Each statement is followed by a line-break.

4.4 Column limit: 80 or 100

Projects are free to choose a column limit of either 80 or 100 characters.
Except as noted below, any line that would exceed this limit must be line-wrapped, as explained in
Section 4.5, Line-wrapping.

Exceptions:

Lines where obeying the column limit is not possible (for example, a long URL in Javadoc,
or a long JSNI method reference).

4.5 Line-wrapping

Terminology Note: When code that might otherwise legally occupy a single line is divided into
multiple lines, typically to avoid overflowing the column limit, this activity is called
line-wrapping.

There is no comprehensive, deterministic formula showing exactly how to line-wrap in
every situation. Very often there are several valid ways to line-wrap the same piece of code.

Tip: extracting a method or local variable may solve the problem without the need to
line-wrap.

4.5.1 Where to break

The prime directive of line-wrapping is: prefer to break at a
higher syntactic level. Also:

When a line is broken at a non-assignment operator the break comes before
the symbol. (Note that this is not the same practice used in Google style for other languages,
such as C++ and JavaScript.)

This also applies to the following "operator-like" symbols: the dot separator
(.), the ampersand in type bounds
(<T extends Foo & Bar>), and the pipe in
catch blocks
(catch (FooException | BarException e)).

When a line is broken at an assignment operator the break typically comes
after the symbol, but either way is acceptable.

This also applies to the "assignment-operator-like" colon in an enhanced
for ("foreach") statement.

A method or constructor name stays attached to the open parenthesis
(() that follows it.

A comma (,) stays attached to the token that
precedes it.

4.5.2 Indent continuation lines at least +4 spaces

When line-wrapping, each line after the first (each continuation line) is indented
at least +4 from the original line.

When there are multiple continuation lines, indentation may be varied beyond +4 as
desired. In general, two continuation lines use the same indentation level if and only if they
begin with syntactically parallel elements.

Section 4.6.3 on Horizontal alignment addresses
the discouraged practice of using a variable number of spaces to align certain tokens with
previous lines.

On both sides of the double slash (//) that
begins an end-of-line comment. Here, multiple spaces are allowed, but not required.

Between the type and variable of a declaration:
List<String> list

Optional just inside both braces of an array initializer

new int[] {5, 6} and
new int[] { 5, 6 } are both valid

Note: this rule never requires or forbids additional space at the start or end of a
line, only interior space.

4.6.3 Horizontal alignment: never required

Terminology Note: Horizontal alignment is the practice of adding a variable number of
additional spaces in your code with the goal of making certain tokens appear directly below certain
other tokens on previous lines.

This practice is permitted, but is never required by Google Style. It is not
even required to maintain horizontal alignment in places where it was already used.

Tip: Alignment can aid readability, but it creates problems for future maintenance.
Consider a future change that needs to touch just one line. This change may leave the
formerly-pleasing formatting mangled, and that is allowed. More often it prompts
the coder (perhaps you) to adjust whitespace on nearby lines as well, possibly triggering a
cascading series of reformattings. That one-line change now has a "blast radius." This can at worst
result in pointless busywork, but at best it still corrupts version history information, slows down
reviewers and exacerbates merge conflicts.

4.7 Grouping parentheses: recommended

Optional grouping parentheses are omitted only when author and reviewer agree that there is no
reasonable chance the code will be misinterpreted without them, nor would they have made the code
easier to read. It is not reasonable to assume that every reader has the entire Java
operator precedence table memorized.

4.8 Specific constructs

4.8.1 Enum classes

After each comma that follows an enum constant, a line-break is optional.

An enum class with no methods and no documentation on its constants may optionally be formatted
as if it were an array initializer:

private enum Suit { CLUBS, HEARTS, SPADES, DIAMONDS }

Since enum classes are classes, all other rules for formatting classes apply.

4.8.2 Variable declarations

4.8.2.1 One variable per declaration

Every variable declaration (field or local) declares only one variable: declarations such as
int a, b; are not used.

4.8.2.2 Declared when needed, initialized as soon as
possible

Local variables are not habitually declared at the start of their containing
block or block-like construct. Instead, local variables are declared close to the point they are
first used (within reason), to minimize their scope. Local variable declarations typically have
initializers, or are initialized immediately after declaration.

4.8.3 Arrays

4.8.3.1 Array initializers: can be "block-like"

Any array initializer may optionally be formatted as if it were a "block-like
construct." For example, the following are all valid (not an exhaustive
list):

4.8.3.2 No C-style array declarations

The square brackets form a part of the type, not the variable:
String[] args, not
String args[].

4.8.4 Switch statements

Terminology Note: Inside the braces of a switch block are one or more
statement groups. Each statement group consists of one or more switch labels
(either case FOO: or
default:), followed by one or more statements.

4.8.4.1 Indentation

As with any other block, the contents of a switch block are indented +2.

After a switch label, a newline appears, and the indentation level is increased +2, exactly as
if a block were being opened. The following switch label returns to the previous indentation
level, as if a block had been closed.

4.8.4.2 Fall-through: commented

Within a switch block, each statement group either terminates abruptly (with a
break,
continue,
return or thrown exception), or is marked with a comment
to indicate that execution will or might continue into the next statement group. Any
comment that communicates the idea of fall-through is sufficient (typically
// fall through). This special comment is not required in
the last statement group of the switch block. Example:

4.8.4.3 The default case is present

Each switch statement includes a default statement
group, even if it contains no code.

4.8.5 Annotations

Annotations applying to a class, method or constructor appear immediately after the
documentation block, and each annotation is listed on a line of its own (that is, one annotation
per line). These line breaks do not constitute line-wrapping (Section
4.5, Line-wrapping), so the indentation level is not
increased. Example:

@Override
@Nullable
public String getNameIfPresent() { ... }

Exception: a single parameterless annotation may instead appear
together with the first line of the signature, for example:

@Override public int hashCode() { ... }

Annotations applying to a field also appear immediately after the documentation block, but in
this case, multiple annotations (possibly parameterized) may be listed on the same line;
for example:

@Partial @Mock DataLoader loader;

There are no specific rules for formatting parameter and local variable annotations.

4.8.6 Comments

4.8.6.1 Block comment style

Block comments are indented at the same level as the surrounding code. They may be in
/* ... */ style or
// ... style. For multi-line
/* ... */ comments, subsequent lines must start with
* aligned with the * on the previous line.

/*
* This is // And so /* Or you can
* okay. // is this. * even do this. */
*/

Comments are not enclosed in boxes drawn with asterisks or other characters.

5.2.2 Class names

Class names are typically nouns or noun phrases. For example,
Character or
ImmutableList. Interface names may also be nouns or
noun phrases (for example, List), but may sometimes be
adjectives or adjective phrases instead (for example,
Readable).

There are no specific rules or even well-established conventions for naming annotation types.

Test classes are named starting with the name of the class they are testing, and ending
with Test. For example,
HashTest or
HashIntegrationTest.

5.2.3 Method names

Method names are typically verbs or verb phrases. For example,
sendMessage or
stop.

Underscores may appear in JUnit test method names to separate logical components of the
name. One typical pattern is test<MethodUnderTest>_<state>,
for example testPop_emptyStack. There is no One Correct
Way to name test methods.

5.2.4 Constant names

Constant names use CONSTANT_CASE: all uppercase
letters, with words separated by underscores. But what is a constant, exactly?

Every constant is a static final field, but not all static final fields are constants. Before
choosing constant case, consider whether the field really feels like a constant. For
example, if any of that instance's observable state can change, it is almost certainly not a
constant. Merely intending to never mutate the object is generally not
enough. Examples:

5.2.6 Parameter names

5.2.7 Local variable names

Local variable names are written in lowerCamelCase, and can be
abbreviated more liberally than other types of names.

However, one-character names should be avoided, except for temporary and looping variables.

Even when final and immutable, local variables are not considered to be constants, and should not
be styled as constants.

5.2.8 Type variable names

Each type variable is named in one of two styles:

A single capital letter, optionally followed by a single numeral (such as
E, T,
X, T2)

A name in the form used for classes (see Section 5.2.2,
Class names), followed by the capital letter
T (examples:
RequestT,
FooBarT).

5.3 Camel case: defined

Sometimes there is more than one reasonable way to convert an English phrase into camel case,
such as when acronyms or unusual constructs like "IPv6" or "iOS" are present. To improve
predictability, Google Style specifies the following (nearly) deterministic scheme.

Beginning with the prose form of the name:

Convert the phrase to plain ASCII and remove any apostrophes. For example, "Müller's
algorithm" might become "Muellers algorithm".

Divide this result into words, splitting on spaces and any remaining punctuation (typically
hyphens).

Recommended: if any word already has a conventional camel-case appearance in common
usage, split this into its constituent parts (e.g., "AdWords" becomes "ad words"). Note
that a word such as "iOS" is not really in camel case per se; it defies any
convention, so this recommendation does not apply.

Now lowercase everything (including acronyms), then uppercase only the first
character of:

... each word, to yield upper camel case, or

... each word except the first, to yield lower camel case

Finally, join all the words into a single identifier.

Note that the casing of the original words is almost entirely disregarded. Examples:

Prose form

Correct

Incorrect

"XML HTTP request"

XmlHttpRequest

XMLHTTPRequest

"new customer ID"

newCustomerId

newCustomerID

"inner stopwatch"

innerStopwatch

innerStopWatch

"supports IPv6 on iOS?"

supportsIpv6OnIos

supportsIPv6OnIOS

"YouTube importer"

YouTubeImporterYoutubeImporter*

*Acceptable, but not recommended.

Note: Some words are ambiguously hyphenated in the English language: for example
"nonempty" and "non-empty" are both correct, so the method names
checkNonempty and
checkNonEmpty are likewise both correct.

6 Programming Practices

6.1 @Override: always used

A method is marked with the @Override annotation
whenever it is legal. This includes a class method overriding a superclass method, a class method
implementing an interface method, and an interface method respecifying a superinterface
method.

6.2 Caught exceptions: not ignored

Except as noted below, it is very rarely correct to do nothing in response to a caught
exception. (Typical responses are to log it, or if it is considered "impossible", rethrow it as an
AssertionError.)

When it truly is appropriate to take no action whatsoever in a catch block, the reason this is
justified is explained in a comment.

Exception: in tests, a caught exception may be ignored without comment if it is
named expected. The following is a very common idiom
for ensuring that the method under test does throw an exception of the expected type, so
a comment is unnecessary here.

The basic form is always acceptable. The single-line form may be substituted when there are no
at-clauses present, and the entirety of the Javadoc block (including comment markers) can fit on a
single line.

7.1.2 Paragraphs

One blank line—that is, a line containing only the aligned leading asterisk
(*)—appears between paragraphs, and before the group of "at-clauses" if
present. Each paragraph but the first has <p> immediately before the first word,
with no space after.

7.1.3 At-clauses

Any of the standard "at-clauses" that are used appear in the order @param,
@return, @throws, @deprecated, and these four types never
appear with an empty description. When an at-clause doesn't fit on a single line, continuation lines
are indented four (or more) spaces from the position of the @.

7.2 The summary fragment

The Javadoc for each class and member begins with a brief summary fragment. This
fragment is very important: it is the only part of the text that appears in certain contexts such as
class and method indexes.

This is a fragment—a noun phrase or verb phrase, not a complete sentence. It does
not begin with A {@code Foo} is a..., or
This method returns..., nor does it form a complete imperative sentence
like Save the record.. However, the fragment is capitalized and
punctuated as if it were a complete sentence.

Tip: A common mistake is to write simple Javadoc in the form
/** @return the customer ID */. This is
incorrect, and should be changed to
/** Returns the customer ID. */.

7.3 Where Javadoc is used

At the minimum, Javadoc is present for every
public class, and every
public or
protected member of such a class, with a few exceptions
noted below.

7.3.1 Exception: self-explanatory methods

Javadoc is optional for "simple, obvious" methods like
getFoo, in cases where there really and truly is
nothing else worthwhile to say but "Returns the foo".

The test methods of a unit test class are perhaps the most common example of this exemption.
These methods can usually be named descriptively enough that no additional documentation is
needed.

Tip:Important: it is not appropriate to cite this exception to justify
omitting relevant information that a typical reader might need to know. For example, for a method
named getCanonicalName, don't omit its documentation
(with the rationale that it would say only /** Returns
the canonical name. */) if a typical reader may have no idea what the term "canonical name"
means!

7.3.2 Exception: overrides

Javadoc is not always present on a method that overrides a supertype method.

7.3.3 Optional javadoc

Classes and members that are not visible outside their package still have Javadoc as
needed. Whenever an implementation comment would be used to define the overall purpose or
behavior of a class, method or field, that comment is written as Javadoc instead. (It's more
uniform, and more tool-friendly.)